Americans have added a quirky step to their voting routine by taking to Twitter and other social media with proof they cast their ballot, amid a drive to boost turnout in the presidential poll.

A study by the Pew Research Centre found that nearly a quarter of registered voters had already announced who they cast their ballot for — or planned to — on social networks flooded with ''I voted'' posts, clicks and photos.

I voted ... Dora Winter of Nampa, Idaho, shows off her sticker after voting.

''Just cast my vote & my grandpa would be so proud!'', @She_Weezy2012 tweeted, posting a picture of an ''I Voted'' sticker on her grey jumper, complete with the #ivoted hashtag, which was being used by thousands of Twitter users.

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Wearing aviator sunglasses and a black cap, singer Lenny Kravitz also waded into the fray, sticking his ''I voted today!'' badge on his finger in a photo on his Twitter account.

In the run-up to Tuesday's election, campaigns, celebrities and others have used social networks extensively to try to persuade people to vote and even beat the record 2008 turnout, when two-thirds of US voters cast a ballot.

Facebook itself posted messages at the top of people's news feeds on Tuesday showing users which friends were voting in the election and urging them to do the same by clicking an ''I'm a voter'' button.

Two-thirds of these were women, according to Facebook statistics, while the top ten most-mentioned terms in the United States on the social network were election-related, with ''vote'', ''Obama,'' ''election'' and ''Romney'' among them.

In its study, the Pew Research Centre found that 22 per cent of respondents in a representative sample of 1011 adults said they had let people know who they voted for, or planned to vote for, on social networking sites.

The survey also found some 25 per cent of Barack Obama supporters had publicly acknowledged their choice, while 20 per cent of Mitt Romney backers had done so.

Nearly one-third of voters, meanwhile, had been encouraged to vote for Obama or his Republican challenger Romney via posts on social media, while one-fifth had tried to convince others to cast their ballot on social networks.

On Twitter, photos abounded of people proudly sporting their ''I voted'' badges and some also posted shots of their ballot papers — which is illegal in some states.

Others put up photos of them voting with their children — an initiative backed by First Lady Michelle Obama, who has encouraged Americans to take their kids to polling stations so they get an idea of the workings of democracy.

Some took to Twitter to point out irregularities. Romney campaign spokesman Ryan Williams posted a photo of a polling station in Philadelphia that had a huge painting of Obama on its wall complete with the word ''hope''.

Electioneering is not allowed at polling places in some states, and Williams later updated followers with a picture of the same Obama mural covered up by several pieces of paper, saying it was still ''not good enough''.

Picswitch, a website that allows users to customise their Twitter profile pictures, had a large selection of logos provided by Obama's campaign team that people were adding to their photos.

The most popular was the generic ''I voted'' banner with Obama's campaign logo, but dozens of people had also chosen a shot of the US president taken from behind, with the caption ''I've got his back.''

Meanwhile, many also checked in on the location-based social network Foursquare to pinpoint exactly where they cast their ballot, and nearly 52,000 photos were posted on photo-sharing platform Instagram with the hashtag #ivoted.

Google also got in on the act, enabling users to click on the search engine's Doodle — made out of ballot papers with its second 'g' falling into a voting box sporting the US flag — to find their nearest polling station.